Why a heavier car is slower everywhere
Why mass costs time in acceleration, braking and cornering alike.
A heavier car is slower everywhere because mass resists any change to its motion, whether that change is speeding up, slowing down, or being pointed in a new direction through a corner, and extra weight makes every single one of those changes cost more effort for the same result.
Three kinds of change, one resistance
Mass is a measure of how much an object resists having its motion changed, and accelerating, braking and cornering are all changes of motion. Speeding a heavier car up takes more work from the engine, and slowing it down takes more work from the tyres and brakes, for the same reason in reverse. Cornering asks for a sideways force strong enough to bend the car's path, and a heavier car resists that bending more, demanding more sideways grip from the same four tyres for the same corner at the same speed.
The tyres make it worse. The grip a tyre supplies rises more slowly than the load pressing on it, so a heavier car both needs more grip and gets a poorer return from its tyres at the higher load, and the two penalties multiply.
Running with a rucksack full of books makes all three costs easy to feel. Getting going from a standstill takes a harder push against the ground. Stopping at a kerb takes more effort through the legs, and the body pitches forward as the extra mass tries to keep travelling. A sharp change of direction needs a harder sideways push through the feet. Packing the same weight tightly against the back makes no difference to any of this, since the penalty comes from the total mass carried.
Where the weight sits, and why teams hunt grams
Placement still matters. A heavier car with its mass carried low and central can corner better than a lighter one with its weight badly placed, but that is a point about distribution sitting on top of the basic rule, and more total mass still costs effort in every phase of driving.
That is why cutting weight is one of the most reliably effective changes a racing team can make. A stronger engine or grippier tyres help in particular phases of a lap, while removing mass helps acceleration, braking and cornering at once. A competitive design therefore starts by asking what does not need to be there at all, and teams weigh individual brackets and fasteners against targets during design, because a few spare grams accepted on hundreds of components add up to a car that is slower everywhere before it has turned a wheel.